code: def test_second_order(self):
      with self.test_session() as sess:
          x = Normal(0.0, 1.0)
          y = 2 * (x**2)
          z = tf.gradients(y, x)[0]
          z = tf.gradients(z, x)[0]
          self.assertEqual(z.eval(), 4.0)
  
file_path: edward/tests/models/random_variable_gradients_test.py
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  import tensorflow
  from edward.models import Bernoulli, Normal
label_desc: Use image node_id 2 for calling the test_random_variable_gradients_class obj's underlying member method code with example usage: obj.test_second_order() without return types